Backlights are used to illuminate thick and thin film displays including liquid crystal displays (LCDs). LCDs with backlights are used in small displays for cell phones and personal digital assistants (PDA), as well as in large displays for computer monitors and televisions. Typically, the light source for the backlight includes one or more cold cathode fluorescent lamps (CCFLs). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (ELP), or one or more hot cathode fluorescent lamps (HCFLs).
The display industry is enthusiastically pursuing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, require adequate idle time to ignite, and require delicate handling. LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. So, displays with LED backlights consume less power than other displays. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display.
With the proliferation of inexpensive LCD displays of various sizes, displays are being used in a multitude of applications. For example, LCD displays are now commonly used in automotive applications in devices such as Global Positioning System (GPS) devices and entertainment systems like televisions and DVD players. A problem facing display manufactures is that the ambient lighting in which many displays are now used is dynamic. Ambient light can cause problems because ambient light intensity and ambient light color, or hue, affect the way a user perceives the image on the display. If the ambient light is very intense, the image on the display may appear dim to the user. If there is abnormally high amount of red light in the ambient light, the image on the display may appear too red to the user.
This problem is highlighted by the example of automotive applications. Both the intensity and hue of the ambient light vary greatly in automotive applications. Moreover, it is not normally feasible to compensate for low ambient light conditions by using an artificial light source since it could reduce the driver's visibility.
Moreover, many devices with LCD displays are portable and may be used in natural ambient light or in artificial ambient light. Natural ambient light and artificial ambient light may have different total intensities and different relative intensities of various hues. For example, incandescent bulbs may have different intensities of green or blue light than a fluorescent bulb. Further, the intensity of different hues of sunlight varies throughout the day. For example, the intensity of the red hue compared to the other hues in sunlight is higher at sunset than at midday in many areas. These variations in hue intensity can affect the perception of display color.
A further challenge for display manufacturers is that the display intensity preferences of different users at different ambient lighting conditions may vary. For example, a first user of a display may prefer a higher intensity at high ambient light intensity conditions than a second user at the same ambient light intensity conditions. Further, the preferences of each user for various display hue intensities may be different at different ambient hue intensities.
Display manufactures have traditionally addressed the above problems with a user intensity control. User intensity controls allow the user to manually increase or decrease the intensity of the display. FIG. 1 is a graphical representation of how this method for controlling display intensity works. A display may have a lower limit on the display intensity. This lower limit, or floor 100, is the lowest intensity the display can achieve. This may be a limitation of the display itself or a limitation of the illumination source or a limitation of driving circuitry for example. For some displays, this floor may be zero intensity or off, but other displays may have a non-zero lower intensity limit. Once the floor is reached, any further attempt to reduce the intensity of the display will not result in a decrease in display intensity. A display may also have an upper intensity limit, or ceiling 101. The intensity ceiling is the highest intensity the display allows. As with the floor, this may be a limitation of the display itself or a limitation of the illumination source or a limitation of driving circuitry for example. Once the intensity ceiling is reached, any further attempt to increase the intensity of the display will not result in an increase in the display intensity. The user intensity setting may also have an upper limit 103 and a lower limit 102. In the region between the floor 100 and ceiling 101 and in between the upper 103 and lower 102 limit of the user intensity setting, an increase in the user intensity setting results in an increase in the display intensity. The user of a display that employs this method must manually increase or decrease the user intensity setting to compensate for changes in perception of the display resulting from ambient light intensity changes.
A limitation of the above approach is that it requires manual adjustment. In display applications in which the ambient light intensity is constant, this limitation may not be significant. However, in applications in which the ambient light intensity is dynamic, this limitation may be significant. In the automotive applications discussed above, a user may have to adjust the display intensity several times throughout the day.
A further limitation of the above approach is that it does not easily compensate for changes in the intensity of hues of the ambient light. In some devices that employ the above approach, the display intensity for individual hues may also be adjusted. However, each hue must be individually adjusted. Adjusting multiple hue intensities several times a day can be cumbersome.
A second approach used by display manufactures to compensate for ambient light intensity changes is to incorporate an ambient light intensity sensor into the display and adjust the display intensity according to the sensor input. An example of this method is illustrated in FIG. 2. The display of FIG. 2 has a ceiling 201 and a floor 200. The ambient light sensor of the display in FIG. 2 has a lower limit 202 and an upper limit 203. The upper and lower limit of the ambient light sensor could be the largest and smallest amount of light detectable by the sensor respectively. When the display intensity is between the floor 200 and ceiling 201 and in between the upper 203 and lower 202 limit of the ambient light sensor, an increase in the ambient light intensity causes the display to increase the display intensity. The increase in display intensity is automatic and does not require user intervention. The increase in display intensity may be continuous 204 or the increase in display intensity may be a step-wise increase 205.
A hybrid approach has also been used. In this approach, the user may set nominal display intensity and a sensor in the display adjusts the display intensity with changes in ambient light intensity according to the method above. In a device that uses a continuous 204 increase, a hybrid approach would result in a correlation 206 between display intensity and ambient light intensity. The correlation 206 is shifted to the right or left from a nominal correlation 204 but still has the same slope, or shape if non-linear, as the original correlation 204.
A limitation of this method is that the correlations 204, 205, 206 between the ambient light and the display intensity are not defined by the user. Therefore, a user may set the nominal display intensity at a first ambient light condition but the resulting display intensity at a second ambient light condition may not be the optimal setting according to the user's preferences.
A further limitation of the above methods is that they do not allow for individual preferences of multiple users.
A further limitation of the above methods is that they do not automatically adjust the intensity of hues of the display based on the intensity of hues of the ambient light. The prior art methods adjust only the intensity of the display backlight and do not vary the hue of the light according to changes in the ambient hue.
The present invention provides innovative systems and methods for automatic display intensity adjustment that solve these limitations.